The present invention relates to electric power meters and particularly to a device for detecting open potential circuits in watthour meters and the like.
Particularly with polyphase watthour or demand meters having more than one potential coil the meter will continue to register power consumption, albeit deficiently, if less than all the potential coils are open circuited. This has enabled theft of power by unscrupulous customers, it not being unknown for a customer to surrepticiously disconnect a potential coil in such manner that it is not detectable by mere visual examination. Even if a potential coil in a polyphase meter should become inoperative due merely to instrument failure, such meter will still appear to visual observation to be operating properly when electrical energy is being used.
Many attempts have been made to provide convenient indication to the meter reader employed by the power company to reveal the meter defect or tampering. U.S. Pat. No. 863,109, issued Aug. 13, 1907 to Simpson et al., describes a relay circuit to be connected in series with the meter for the stated purpose "to prevent tapping of the circuits by maliciously-disposed persons, to detect defective installation or wiring, or other diversion from such measuring instruments of the currents designed to pass through them." Other patents disclose meters provided with auxiliary magnetically actuated mechanical flags for indicating an open circuit. In fact, a large body of prior art exists where some form of detection mechanism or means is incorporated within the meter. However, in the same manner that access is obtained to the meter interior for disconnecting a potential coil, the detection mechanism can be thwarted.
An alternative approach using a device carried by the meter reader is the basis of Phillips U.S. Pat. No. 4,583,043, issued Apr. 15, 1986, entitled "Determination of Electrical Energy Diversion". This device employs two clip-on transformers, one for connection about an inlet power cable from the electric utility and the other about an inlet power supply to the consumer. Electric current measuring means determines any difference in the current sensed by the two transformers. Unfortunately, the Phillips device requires the individual power conductors to be accessible and this generally is not the case, the conductors normally being enclosed within a cable containing the return conductor. A transformer around such cable will detect zero current.
A company named KC Industries, Inc., whose address is Box E, Cambridge, Nebr. 69022, has produced a magnetic flux indicator which, according to the advertising copy of the manufacturer, was designed to enable polyphase meters to be easily checked without removing the meter or making any connections to the meter. The flux indicator employs a mechanical analog movement in a small case. The operating instructions in the advertisement reads: "Simply hold the `Magnetic Flux Indicator` level and parallel to the outer surface of the meter case, with either side against the glass meter cover (the unit does not have to be removed from its protective carrying case to be used, if it is ordered with this option). Rotate the MFI around the meter until a potential coil is located. A deflection of the indicator means the coil is energized, the absence of a deflection denotes an open or faulty coil. Both potential coils should be checked in this manner to ensure that they are properly energized. As the potential coils may vary in mounting location in various meters, the amount of deflection will vary from meter to meter. Thus, it is not the magnitude of the deflection which is important, but simply whether or not there is a deflection."
While the "Magnetic Flux Indicator" described above can detect open coils, it has certain limitations inherent in its use of a mechanical analog indicator. A serious drawback is encountered when the energy or demand meter is located above shoulder level because the device is orientation sensitive, and the meter movement becomes difficult if not impossible to read when the energy meter is located high up. Also, the meters are not always found in a well illuminated location, and holding a flashlight makes it even more difficult to use when the meters are mounted at a higher elevation from the floor. Another problem arises from the analog indication. Unless the meter reader is trained especially in the use of the KC Industries, Inc. device, there is considerable room for error. An absence of a deflection of the indicator could be due to an improper positioning or orientation of the device and not due to an inoperative voltage coil.